Astronomy & Astrophysics

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$\begin{figure} \par\mbox{\includegraphics{aage271f1a.eps}\hspace{5mm} \includegr... ...age271f1b.eps}\hspace{5mm} \includegraphics{aage271f1d.eps} } \par \end{figure}$ Figure 1: a) Reaching six times deeper than 2MASS is this ${\it JHK}_{\rm s}$ image of M 33, with a simple ellipsoid model subtracted. A gargantuan plume-like ring of red stars stretches in a swath (up to 5' in width) for over 120 degrees, commencing at $\sim$ 14' north of the galaxy centre. A fainter counterpart is found to the south. The surface brightness of the northern arc is only $\sim$ 20-21 mag arcsec^-2 at 2.2 $\mu$ m, which explains why it has hitherto not been discussed in previous near-infrared studies. An ellipsoidal swath is selected to pass through the plumes; that sector passing through the prominent northern plume is color-coded red. b) As in a), but deprojected, beautifully reveals the plumes as well as the inner arms. The northern plume is labelled A to B. c) A J-Ks image of M 33, coded blue-green-red in order of increasingly red J-Ks values, from 0.5 to 1.5. The outer arcs (colored green and red) appear to form a partial ring. It is tempting to liken this ring to the one recently reported in the outer disk of our Milky Way (Ibata et al. 2003). d) Fourier spectra generated from the deprojected near-infrared $K_{\rm s}$ mosaic of M 33. The dominant m=2 mode in M 33 does not arise from the inner pair of spiral arms seen in Fig. 1a, but from the giant outer ring or arcs of stars identified in this study: a prominent plume in the north, and a fainter counterpart to the south. While M 33 in the optical shows ten spiral arms (Sandage & Humpheys 1980), the galaxy in the near-infrared has only two low-order modes: m=1 and m=2.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{block_fig2.eps} \end{figure}$ Figure 2: An overlay of the integrated cluster light in the Magellanic Clouds (from Fig. 4 of Persson et al. 1983), with the integrated colors for plume stars in the NE sector (green dots) and SW sector (red dots) of M 33. The open symbols principally show the Searle, Wilkinson & Bagnuolo (1980) (SWB) Magellanic cluster types I, II and III; these do not contain carbon stars. The filled symbols are SWB types V, VI and VII. The red Magellanic carbon-star-bearing clusters lie above and to the right of the black dashed lines, in the Magellanic "IR-enhanced'' cluster regime identified by Persson et al. (1983); these are predominantly SWB types V and VI. Red bars show our 1-sigma uncertainty in the color for each point in this figure; the black error bars are from the Persson et al. (1983) analysis. The thin blue dotted and solid lines show the Bessell & Brett (1998) tracks for giants and dwarfs respectively. The overlay provides strong evidence for a carbon star population in the M 33 plumes.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{block_fig3_new.eps} \end{figure}$ Figure 3: A J-H, H-Ks color-color diagram contrasting the nuclear colors with those of northern inner spiral arm and the NE and SW plume populations. The colors were derived from the integrated flux of regions centered on the N inner arm and NE and SW plumes. The photometry consisted of circular, 10 arcsec radius aperture photometry carried out on the 2MASS J, H and Ks images, cleaned of foreground stars and corrected for disk inclination. The evolved giant and main-sequence dwarf tracks are shown with green dashed and solid lines, following Bessell & Brett (1998).
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{block_fig4_new.eps} \end{figure}$ Figure 4: Observations with the Hale 5m reflector, through a sector of the northern red arc. The sample has been cleaned of all low detection sources, with a S/N < 10. The extinction reddening vector is indicated with the red arrow. The evolved giant and main-sequence dwarf tracks are from Bessell & Brett (1998). In this color-color plot, no foreground MW stars were statistically removed, as in the other color-color plot. Here foreground stars appear with blue colors ( $H-K_{\rm s} < 0.2$ and J-H < 0.8; $J-K_{\rm s} < 1.0$ ), while most of the M 33 sources have colors that are redder than $H-K_{\rm s} = 0.4$ and J-H = 0.9 ( $J-K_{\rm s} = 1.3$ ) mag. The color uncertainties for the red sources are less than 10%.
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In the text